Construction of gas pipelines and petroleum pipelines has been promoted as the basis of energy supply. In recent years, many gas fields remote from consuming regions are developed under trends of increasing demand for natural gas. Thus, recent pipelines tend to be long and have large diameters and high pressures for large-scale transportation.
Such new pipelines use high-strength steel pipes since they are required to withstand high inner pressure despite large diameters and small pipe thickness. This is because, by reducing the pipe thickness, the cost for welding on site and cost for transporting pipes can be reduced, and the total cost of pipeline construction and operation can thus be reduced.
Although steel pipes can fully take advantages of ductility of the raw material under tensile load, local buckling occurs under compressive load since their shape of cross-section is cylindrical and has a small thickness. Whereas uniform elongation is about 10%, critical strain under compressive load is about 1 to 2%; hence, in pipeline plastic design, the critical strain is highly likely to be a controlling factor. In particular, steel pipes with small thickness tend to decrease critical strain and it is important to increase the critical strain.
Accordingly, the following proposals have been made to increase the critical strain and to thereby enhance the buckling performance.
That is, a tensile test piece taken by making the longitudinal direction of the test piece to correspond to the axial direction of a steel pipe is used to conduct tensile testing. A steel pipe having a positive nominal stress/nominal strain gradient in the observed nominal stress-nominal strain curve at any amount of strain from the yield point to 5% on-load strain amount exhibits a notably high outer diameter/pipe thickness ratio, i.e., the limit at which the local buckling occurs, when compared to a steel pipe having zero or a negative nominal stress/nominal strain gradient in the same range, and thus does not easily cause local buckling. On the basis of these findings, a steel pipe is adjusted such that the nominal stress/nominal strain gradient at any strain from the yield point to the 5% on-load strain is positive in the nominal stress/nominal strain curve observed by the tensile testing in the axial direction (refer to Patent Document 1).    Patent Document 1: Japanese Unexamined Patent Application Publication No. 9-196243